2
Usergroup membership is updated on login/logout and saved in a session variable. It is therefore not a simple task to retrieve just the user(s) you put in a new group and manipulate their session variables.
Look at the sessions table in the database and if you cannot figure out how to manipulate it, you should not do it (sorry - that may sound harsh. What I mean is that if it is not apparent to you what to do (and it is not that simple) then I would not recommend that you engage into it. Session variables are PHP stuff and not XOOPS stuff and should not be messed with by people, who are not fully aware of what they are doing)